Staff: Mentor

What is the significance of UV vs IR, in terms of wavelength/frequency and energy?

Light of different energies will have different transmission capability. Highest energy photons like X-rays (think of hospital X-rays) and gamma rays can penetrate solid materials, with the depth of penetration increasing with energy.

Photons of shorter wavelengths are more energetic, and therefore can induce chemical transitions and disrupt bonds between molecules, damaging biological tissue. I think penetration depth isn't the real issue, as the cross section usually goes up with energy. Therefore, IR probably penetrates skin deeper than UV, (see also greenhouse effect), whereas other materials it would be different.

The key issue is that IR photons are only energetic enough to excite vibrational degrees of freedom (causing temperature rise), whereas UV photons can excite atomic transitions and break bonds. X-rays will do that and gamma rays can even cleave nuclei (spallation)

I believe infared and microwaves have longer wavelengths and thus less frequency than UV light does. That means the photons in the infared don't carry as much energy in them as the photons in UV light do.

Ibrits has it correct as far as saying how UV can excite electrons in different atoms enough to break bonds via the Photoelectric effect. X-rays and gamma rays (especially) are way more harmful to contact with the human skin and body and can casue mutation of genes even.